The new 100-400 does indeed exist and is being actively tested, however Canon is in no rush to announce the lens. We’re told that the current 100-400 still sells at an amazing clip and outpaces the 70-300L by a 4 to 1 margin.

Canon may also be waiting to see how the EF 200-400 f/4L IS 1.4x sells, would people really hold off buying it if a new 100-400 was available?